Tunable Progressive Cavity Pump

ABSTRACT

A well pump assembly includes a progressive cavity pump having a stator with an elastomeric inner portion. The stator has an axial cavity with internal lobes; a rotor with external lobes positioned within the axial cavity. An effector selectively increases and decreases a stiffness of the stator by changing a cross sectional area of the axial cavity in the stator. The effector may include a reservoir within the stator containing a fluid. A reservoir pump selectively increases and decreases a pressure of the fluid in the reservoir in response to sensing the flow rate from the progressive cavity pump and the torque of the motor. Alternately, the reservoir may contain a magneto-rheological fluid (MR fluid). A coil generates an electromagnetic field within the MR fluid to selectively increase and decrease a viscosity of the MR fluid.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to provisional application 61/878,367, filed Sep. 16, 2013.

FIELD OF THE DISCLOSURE

This disclosure relates in general to progressive cavity pumps for wells and in particular to a system that changes the inner diameter of the stator in response to changes in operating conditions.

BACKGROUND

One type of well pump used in oil wells is a progressive cavity pump. The pump has a stator with an elastomeric inner portion. An axial cavity having an internal helical profile extends through the stator. A rotor with an external helical profile fits within the axial cavity. A motor causes the rotor to rotate, with the interaction of the helical profile on the rotor and the helical profile in the stator causing fluid to be pumped upward through the cavity. The rotation of the rotor also causes the rotor to orbit within the stator.

The interface between the rotor and axial cavity is sensitive and may change due to various conditions in the well. The stator may swell, causing the interference between the rotor and the helical profile of the axial cavity to create excessive friction, increasing the torque and creating a potential to lock or break of the rotor. On the other hand, if the stator shrinks, the cross-sectional area of the axial cavity increases, reducing the interference between the rotor and the axial cavity. Erosive wear may also increase the cross-sectional area of the axial cavity. If too large, the interface between the rotor and the stator may allow leakage of well fluid, reducing the efficiency of the pump.

The radial shrinkage or swelling of the stator depends on well fluids and environmental conditions. For example, the hydrocarbon content of the well fluid may cause the stator to swell, decreasing the cross-sectional area of the axial cavity while the pump is being lowered into the well. Consequently, manufacturers custom size the interference between the rotor and the axial cavity for a particular well. However, if the environmental conditions change, the axial cavity geometry may cause the pump to either become less efficient or cease to function.

SUMMARY

A well pump assembly includes a progressive cavity pump having a stator with an elastomeric inner portion. The stator has an axial cavity with an internal helical profile. A rotor with an external helical profile is positioned within the axial cavity. A motor operatively coupled to the progressive cavity pump rotates the rotor when supplied with power. At least one effector is cooperatively associated with the stator to selectively increase and decrease a stiffness of the stator. Preferably, a controller senses operating conditions of the progressive cavity pump assembly and controls the effector in response. The change in stiffness may be caused by the effector increasing and decreasing a cross sectional area of the axial cavity in the stator.

The effector may comprises a reservoir within the stator separate from the axial cavity and containing a pressure fluid. A reservoir pump for selectively increases and decreases a pressure of the pressure fluid in the reservoir. The reservoir may be elongated and extend along a length of the stator, separated from the axial cavity.

Alternately, the stator may contain a reservoir filled with a magneto-rheological fluid (MR fluid). A coil generates an electromagnetic field within the MR fluid to selectively increase and decrease a viscosity of the MR fluid. The MR fluid reservoir may have two portions axially spaced apart and connected by an orifice. A coil generates an electromagnetic field within the MR fluid at the orifice to selectively increase and decrease a viscosity of the MR fluid.

The pump assembly may have a plurality of separate effectors spaced along a length of the progressive cavity pump. Each of the effectors is separately controllable for varying a stiffness of the stator along the length of the progressive cavity pump.

BRIEF DESCRIPTION OF THE DRAWINGS

The present technology will be better understood on reading the following detailed description of nonlimiting embodiments thereof, and on examining the accompanying drawings, in which:

FIGS. 1A and 1B are a sectioned side view, partially schematic, of a progressive cavity well pump assembly having a stator with tunable features in accordance with this disclosure.

FIG. 2 is an enlarged transverse cross-sectional view of an alternate embodiment of the pump of FIG. 1A.

FIG. 3 is an enlarged axial cross-sectional view of the pump of FIG. 1A with the rotor not shown.

FIG. 4 is an axial cross-sectional view of an alternate embodiment of the pump of FIG. 3.

FIG. 5 is an enlarged axial cross-sectional view of another alternate embodiment of the pump of FIG. 1A.

FIG. 6 is a schematic, perspective exploded view of the pump of FIG. 5.

FIG. 7 is a schematic, perspective exploded view of a portion of the effector of the pump of FIG. 5.

DETAILED DESCRIPTION

Referring to FIG. 1A, a cased well 11 has a wellhead assembly or production tree 13 mounted at its upper end. Production tree 13 is shown schematically and has a flow line 15 for discharging production fluid from well 11. A valve 17 opens and closes flow line 15. Preferably the surface equipment includes a flow meter 19 connected into flow line 15 for measuring the flow rate of the well fluid. Alternately, flow meter 19 could be located in the well. An electrical line 21 connects flow meter 19 to a controller 23 located on the surface adjacent production tree 13.

Production tubing 25 has an upper end supported by a hanger (not shown) in production tree 13 and extends into cased well 11. Tubing 25 may comprises joints of pipe secured by threads to each other. Alternately, tubing 25 could be continuous coiled tubing deployed from a reel.

A progressive cavity pump 27 secures to a lower end of tubing 25 to pump well fluid up to production tree 13. Alternately, progressive cavity pump 27 could be deployed through tubing 25. Pump 27 has a stator 31 within a cylindrical housing 29, which may be considered to be part of stator 31. Stator 31 is fixed against rotation in housing 29, and at least an inner portion is formed of an incompressible but resilient elastomeric material. Stator 31 has an axial cavity 33 extending its length that is formed with a helical configuration. In FIGS. 1A and 3, axial cavity 33 has two helical lobes, creating a sinusoidal appearance, narrowing and widening with inward projecting lobes separated by outward extending valleys. Axial cavity 33 could have more than two helical lobes, such as stator 31′ in FIG. 2, which has an axial cavity 33′ with three helical lobes.

A rotor 35 rotatably extends through stator axial cavity 33. Rotor 35 is normally of metal and has an exterior profile 37 that slidingly engages the profile of axial cavity 33. Exterior profile 37 has a single helical configuration that is also sinusoidal in appearance. However, when viewed in cross-section, the lobes appear on one side of rotor 35 to be offset from the lobes on the opposite side, presenting a sinuous appearance. The transverse cross-sectional appearance of rotor 35 is illustrated by rotor 35′ in FIG. 2.

Exterior profile 37 and the profile of axial cavity 33 are well known and conventional. Because of exterior profile 37 and the profile of axial cavity 33, when rotor 35 rotates, it orbits around axis 39 of pump housing 29. As rotor 35 rotates, an interference fit with axial cavity 33 causes rotor 35 to deflect or deform elastomeric stator 31 inward and outward as well fluid is pushed upward into tubing 25.

A gripping section 40 may be mounted to the upper end of rotor 35 to be engaged by a tool for retrieving rotor 35 from stator 31. Normally, the upper end of rotor 35 extends above stator 31, and the lower end of rotor 35 extends below stator 31.

The interface between rotor 35 and axial cavity 33 is sensitive and may change due to various conditions in the well. Stator 31 may swell, causing the interference between rotor 35 and the profile of axial cavity 33 to create excessive friction, increasing the torque and creating a potential to lock or break of rotor 35. On the other hand, if stator 31 shrinks, the cross-sectional area of axial cavity 33 increases, reducing the interference between rotor 35 and axial cavity 33. Erosive wear may also increase the cross-sectional area of axial cavity 33. If too large, the interface between rotor 35 and axial cavity 33 may allow leakage of well fluid, reducing the efficiency of pump 27.

The radial shrinkage or swelling of stator 31 depends on well fluids and environmental conditions. For example, the hydrocarbon content of the well fluid may cause stator 31 to swell, decreasing the cross-sectional area of axial cavity 33 while pump 27 is being lowered into the well. Consequently, manufacturers custom size the interference between rotor 35 and axial cavity 33 for a particular well. However, if the environmental conditions change, the axial cavity geometry may cause the pump to either become less efficient or cease to function.

To avoid these problems, an effector is employed that selectively increases and decreases the stiffness of elastomeric stator 31 in response to changes in operating conditions. A change in stiffness also changes the interference between rotor 35 and axial cavity 33. The effector may also reduce the cross-sectional area of the axial cavity, which in effect, changes the stiffness of stator 31.

Referring to FIG. 3, in this example, an effector chamber or reservoir 41 within pump housing 29 is formed within or outside of stator 31. In this example, reservoir 41 comprise several separate axially extending cavities, each formed within stator 31 and evenly spaced around axis 39. Each reservoir 41 could have an axis parallel with axis 39, or each reservoir could be helical and extend helically around axis 39. Alternately, reservoir 41 could be annular, extending completely around an outer diameter of stator 31. Effector reservoir 41 may be elongated, as shown, and could extend all or just part of the length of stator 31. A pressure fluid 43 pumped by a reservoir pump or compressor 45 selectively increases and reduces fluid pressure within reservoirs 41. Pressure fluid 43 may be incompressible, such as a hydraulic fluid. Pressure fluid 43 may alternately be a compressible fluid, such as air. Pressure fluid 43 within each reservoir 41 is isolated or blocked from fluid communication with well fluid in axial cavity 33.

Reservoir pump 45 may be located adjacent to production tree 13 and controller 23 (FIG. 1A). Controller 23 (FIG. 1A) controls reservoir pump 45 based on torque sensed and the flow rate of well fluid being monitored by flow meter 19. As an alternate to being mounted adjacent to production tree 13, the portion of controller 23 that controls reservoir pump 45 could be mounted to progressive cavity pump 23 within the well. Reservoir pump 45 draws fluid 43 from a tank 47. A valve 48 allows reservoir pump 45 to pump fluid 43 to reservoirs 41 and will hold the pressure when reservoir pump 45 is turned off. When actuated by controller 23, valve 48 allows flow back of fluid 43 to tank 47.

When the pressure of fluid 43 increases, reservoirs 41 expand and stiffen stator 31. If rotor 35 is not present, as shown in FIG. 3, the increase in fluid pressure in reservoirs 41 causes the dimensions of axial cavity 33 to shrink, as indicated by the dotted lines 49. The flow area of axial cavity 33 thus shrinks. The difference between the unaltered size of axial cavity 33 and the reduced size shown by the dotted lines may only be 0.20 inches or less, as an example. During operation, rotor 35 (FIG. 1) will be present, and being metal, it does not change dimensions in response to increasing pressure in reservoirs 41. Thus the interference between rotor 35 and axial cavity 33 increases in response to increasing fluid pressure within reservoirs 41.

Referring to FIG. 1A, rotor 35 may be driven in various conventional manners. In this example, a flex shaft 51 couples to a lower end of rotor 35 via a coupling 53 that allows rotor 35 to stab into engagement with flex shaft 51. Flex shaft 51 rotates within a connector shaft housing 55 that has a well fluid intake 57 for admitting well fluid to axial cavity 33. A concentric coupling 59 connects to and causes the lower end of flex shaft 51 to remain concentric on axis 39. The upper end of flex shaft 51 and coupling 53 orbit. Flex shaft 51 is typically formed of a steel material.

A drive shaft 61 has an upper end that connects to concentric coupling 59. Drive shaft 61 extends through a seal section 63. In this example, a gear reducer 65 secures to the lower end of seal section 63 to reduce the rotational speed of drive shaft 61. An electrical motor 67 couples to the lower end of gear reducer 65. Motor 67 may be a three-phase type that rotates typically around 3600 rpm. Motor 67 has a drive shaft (not shown) that couples to gear reducer 65 for rotating drive shaft 61 at a lower rate of speed. A dielectric lubricant fills motor 67 and also part of seal section 63. Seal section 63 reduces a pressure differential between well fluid on the exterior and the lubricant within motor 67. Seal section 63 may be a conventional type having a communication port that admits well fluid to one side of a bag or bellows, the other side being in contact with the lubricant. A power cable 69 connects to motor 67 and extends alongside tubing 25 to the surface where it connects to controller 23. Optionally, a sensing unit 71 may connect to motor 67. Sensing unit 71 senses various parameters such as temperature and well fluid pressure.

Pump 27 may alternately be driven by a motor located adjacent production tree 13. In that case, a drive rod (not shown) extends from the surface motor to pump 27.

In operation, controller 23 supplies electrical power to motor 67, which causes rotor 35 to rotate, pumping well fluid up tubing 25 to production tree 13. Controller 23 monitors the flow rate with flow meter 19. Controller 23 also monitors the torque required to rotate rotor 35. Torque monitoring can be accomplished various ways. In one example, controller 23 monitors the electrical current supplied via power cable 69 to motor 67. Controller 23 will actuate reservoir pump 45 to increase the pressure of fluid 43 in reservoirs 41 if the flow rate drops below an acceptable level. Controller 23 will stop reservoir pump 45 from increasing the fluid pressure in reservoirs 41, and with valve 48, hold the desired pressure once a desired flow rate is reached. Controller 23 will also control valve 48 to bleed off pressure in reservoirs 41 if the torque monitored is too high.

The initial interference between rotor 35 and stator axial cavity 33 could be sized loosely enough so that once pump 27 has been located in the well, the start up torque will not be excessive. That is, possible swelling of stator 31 could be accounted for in advance by making the dimensions of stator axial cavity 33 sufficiently large so that expected swelling would not cause too much interference between stator 31 and rotor 35. When pump 27 is first installed, reservoir pump 45 would not be operating, and the pressure of fluid 43 in reservoirs 41 would be equal to the hydrostatic pressure of the well fluid in the well. After pump 27 operates for a selected duration, controller 23 may increase the stiffness of stator 31 by causing reservoir pump 45 to increase the pressure of fluid 43 in reservoirs 41, thereby increasing the flow rate of well fluid. If the torque becomes too high, controller 23 actuates valve 48 to bleed off some of the pressure in reservoirs 41. Controller 23 thus continually tunes pump 27 to operate with a desired stiffness of stator 31. As an alternate to automatic control by controller 23 based on torque and flow rate, the operator could manually adjust the stiffness of stator 31 with manual controls on controller 23 to change the pressure within reservoirs 41.

Referring to FIG. 4, progressive cavity pump 27′ has more than one stator portion, and three portions are shown by the numerals 31 a, 31 b, and 31 c. Stator portions 31 a, 31 b, 31 c are shown stacked coaxially on each other within a single housing 29′, however they could have separate housings secured to each other. Each stator portion 31 a, 31 b, and 31 c has one or more reservoirs 41 a, 41 b and 41 c, respectively. A separate flow line 74 a, 74 b and 74 c leads from the reservoirs 41 a, 41 b and 41 c. A separate valve 48 a, 48 b and 48 c is located in each flow line 74 a, 74 b and 74 c, respectively. In this example, a single reservoir pump 45 (FIG. 3) supplies pressure fluid through the separate valves 48 a, 48 b and 48 c to separate flow lines 74 a, 74 b, and 74 c. Flow lines 74 a, 74 b and 74 c are illustrated on the exterior of housing 29′, but they could extend upward through the stator portions 31 a, 31 b and 31 c to an upper end of progressive cavity pump 27′. The pressure fluid in each reservoir 74 a, 74 b and 74 c is isolated or not in fluid communication with the pressure fluid in the other reservoirs 74 a, 74 b and 74 c.

During operation of the embodiment of FIG. 4, the well fluid pressure within stator cavity 33′ caused by the rotation of the rotor (not shown) gradually increases from the bottom to the top of progressive cavity pump 27′. Because of the lower pressure within lower stator portion 31 a, the desired stiffness of lower stator portion 31 a may be less than the desired stiffness in intermediate stator portion 31 b. Similarly, the optimum stiffness in intermediate stator portion 31 b may be less than the optimum stiffness of upper stator portion 31 c. More interference and stiffness may be desirable in the portions of pump 27′ having higher fluid pressures. Controller 23 (FIG. 1A) can control pump 45 and valves 48 a, 48 b and 48 c to provide a different reservoir fluid pressure in each reservoir 41 a, 41 b and 41 c. Alternately, an operator could manually control valves 48 a, 48 b and 48 c to maintain different pressures in reservoirs 41 a, 41 b and 41 c.

Although three separate stator portions 31 a, 31 b and 31 c are illustrated, pump 27′ could have more or fewer. Also, rather than separate stator portions, a single stator could have several zones along its length, each zone having a separate reservoir.

Referring to FIGS. 5-7, in this embodiment, two separate stator sections 73 a, 73 b are illustrated, but more could be employed. Stator sections 73 a, 73 b are axially aligned along a longitudinal axis 75 and spaced axially apart from each other a short distance. Referring more particularly to FIG. 5, each stator section 73 a, 73 b is of incompressible elastomeric material fixed for non rotation within a steel housing 77. The ends of housings 77 may protrude past the ends of stator sections 73 a, 73 b and abut each other. Each stator section 73 a, 73 b has an axial cavity 79 for receiving a conventional rotor 80, which is a single-piece member extending through both stator sections 73 a, 73 b.

A stator stiffness effector 81 is mounted between opposing ends of stator sections 73 a, 73 b. Effector 81 has a rigid tubular body 83 with one end abutting stator section 73 a and the other end abutting stator section 73 b. Body 83 has an axial bore 85 that is cylindrical and has a diameter large enough so that rotor 80 does not contact it as rotor 80 rotates and orbits. Effector body 83 has at least one, and preferably several magneto rheological (MR) passages 87. In this example, three MR passages 87 are shown in FIG. 7, spaced equally around axis 75. Each MR passage 87 has a first or upper section 87 a and a second or lower section 87 b. Each section 87 a, 87 b joins a central pocket 88 formed in effector body 83. In this example, effector body 83 has three pockets 88.

Mating MR fluid reservoirs 89 are formed within stator sections 73 a, 73 b to register with MR passages 87. Each MR fluid reservoir 89 may have the same diameter as each MR passage 87. Seals (not shown) seal the interface between MR passages 87 and MR fluid reservoirs 89. Each MR fluid reservoir 89 extends parallel to axis 75 a selected distance and has a closed end opposite the end joining MR passages 87. The axial length of each MR fluid reservoir 89 need not be as long as each stator section 73 a, 73 b, but could be. MR fluid reservoirs 89 a are located in stator section 73 a and mate with MR passage sections 87 a. MR fluid reservoirs 89 b are located in stator section 73 b and mate with MR passage sections 87 b.

An orifice or tube 91 extends through each pocket 88 and connects each MR fluid passage 87 a with the corresponding MR fluid passage 87 b. Orifice tube 91 seals to MR fluid passages 87 a, 87 b and has a flow area smaller than the flow areas of MR fluid passages 87 a, 87 b, creating an orifice.

A magneto rheological (MR) fluid 93 is located in MR reservoirs 89, MR fluid passages 87 and orifice tubes 91. MR fluid 93 is a known liquid that will undergo a significant change in viscosity when an electromagnetic field passes through MR fluid 93. One or more coils or electromagnets 95 are located within each pocket 88 adjacent to each orifice tube 91 to impose an electromagnetic field on MR fluid 93 contained in orifice tube 91. In this example, two substantially flat electromagnets 95 are located in each pocket 88, one or each side of orifice tube 91. Electromagnets 95 are connected by wires (not shown) to a controller, such as controller 23 (FIG. 1) to selectively supply electrical current.

Stator sections 73 a, 73 b may be secured together with effector 81 sandwiched between in various manners. If desired, effectors 81 could also be located at the upper end of stator section 73 a and lower end of stator section 73 b. A collar or clamp 99 is schematically illustrated as enclosing effector 81 and joining stator housings 77. Effector body 83 may have an outer diameter smaller than the inner diameter of housings 77, as illustrated, and fits within the portions of housings 77 that extend beyond stators 73 a, 73 b. Rather than a collar 99, the abutting ends of housings 77 could be welded to each other or secured in other manners.

During operation of the embodiment of FIGS. 5-7, rotation of rotor 80 exerts radial outward forces on each stator section 73 a, 73 b, causing lobes within axial cavity 79 to deflect radially back and forth. The deflection force transmits through stator sections 73 a, 73 b and acts radially on MR fluid reservoirs 89 a, 89 b, alternately squeezing and relaxing reservoirs 89 a, 89 b. This alternating force on MR fluid reservoirs 89 a, 89 b causes a pumping action of MR fluid 93, causing it to flow in an oscillating manner through orifice tubes 91. At the same time, the rotation of rotor 80 pumps well fluid through axial cavity 79 up from stator section 73 a.

If controller 23 (FIG. 1A) senses from flow meter 19 that the flow rate of well fluid is too low, it will send a signal to electromagnets 95, which impose an electromagnetic field on MR fluid 93 flowing through orifice tube 91. The viscosity of MR fluid 93 within each orifice tube 91 increases as a result, which slows the flow rate between MR fluid reservoirs 89 a, 89 b. The fluid pressure within reservoirs 89 a, 89 b increases as the helical lobes of rotor 80 exert radial outward forces on stator sections 73 a, 73 b. The increased pressure resists the outward deflection of stator sections 73 a, 73 b, thereby increasing the stiffness of stator sections 73 a, 73 b. The increased stiffness effectively increases the interference between rotor 80 and stator sections 73 a, 73 b, thereby increasing the flow rate.

If controller 23 senses that the torque to rotate rotor 80 is too high, it will cut off the voltage supplied to electromagnets 95. The viscosity of MR fluid 93 within orifice tubes 91 rapidly drops, lowering the pumping pressure within MR fluid reservoirs 89. The stiffness of stator sections 73, 73 b thus decreases to reduce the torque. Rather than automatically controlling the stiffness with controller 23 based on torque and well fluid flow, an operator could manually vary the stiffness with manual controls on controller 23 to supply voltage to electro magnets 95.

The embodiment of FIGS. 5-7 could also be incorporated into separate zones, in a manner similar to the embodiment shown in FIG. 4. Controller 23 (FIG. 1A) could supply voltages to electromagnets 95 in one or more of the zones to increase or decrease the viscosity and not to other of the zones. Also, the zones could be manually controlled.

The foregoing aspects, features, and advantages of the present technology will be further appreciated when considered with reference to the following description of preferred embodiments and accompanying drawings, wherein like reference numerals represent like elements. In describing the preferred embodiments of the technology illustrated in the appended drawings, specific terminology will be used for the sake of clarity. However, it is to be understood that the specific terminology is not limiting, and that each specific term includes equivalents that operate in a similar manner to accomplish a similar purpose.

For example, other effectors to increase and decrease the stiffness of the stator in response to changing conditions are feasible. Shape memory gel and shape memory alloys change shapes in response to voltage changes. Piezoelectric crystals, voice coils or any other media or elements that alter geometry in response to changing conditions sensed could also be used.

Although the technology herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present technology. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present technology. 

1. A well pump assembly, comprising: a progressive cavity pump having a stator with an elastomeric inner portion, the stator having an axis, an axial cavity with an internal helical profile, and a rotor with an external helical profile positioned within the axial cavity; a motor operatively coupled to the progressive cavity pump for rotating the rotor when supplied with power, and at least one effector cooperatively associated with the stator that selectively increases and decreases a stiffness of the stator.
 2. The assembly according to claim 1, further comprising: a controller that senses operating conditions of the progressive cavity pump assembly and controls the effector in response.
 3. The assembly according to claim 1, wherein the effector selectively increases and decreases a cross sectional area of the axial cavity in the stator.
 4. The assembly according to claim 1, wherein the effector comprises: a reservoir within the stator separate from the axial cavity and containing a fluid; and a reservoir pump for selectively increasing and decreasing a pressure of the fluid in the reservoir.
 5. The assembly according to claim 1, wherein the effector comprises: at least one elongated reservoir extending along a length of the stator, separated from the axial cavity and containing a fluid; a reservoir pump located exterior of the progressive cavity pump that is connected to the reservoir by a fluid line; a valve for opening and closing the fluid line; and a controller that senses a flow rate from the progressive cavity pump and a torque from the motor and controls the valve and the reservoir pump in response.
 6. The assembly according to claim 1, wherein the effector comprises: at least one fluid reservoir within the stator separate from the axial cavity; a magneto-rheological fluid (MR fluid) within the reservoir; and a coil that generates an electromagnetic field within the MR fluid to selectively increase and decrease a viscosity of the MR fluid.
 7. The assembly according to claim 1, wherein the effector comprises: at least one reservoir within the stator having two portions axially spaced apart and connected by an orifice, the reservoir being separated from the axial cavity; a magneto-rheological fluid (MR fluid) within the reservoir; a coil that generates an electromagnetic field within the MR fluid at the orifice to selectively increase and decrease a viscosity of the MR fluid; wherein rotation of the rotor deforms the internal helical profile, causing the MR fluid to flow through the orifice between the portions of the reservoir; and increasing the viscosity of the MR fluid slows a flow rate of the MR fluid through the orifice and thereby stiffens the stator.
 8. The assembly according to claim 1, wherein the at least one effector comprises: a plurality of separate effectors spaced along a length of the progressive cavity pump, each of the effectors being separately controllable for varying a stiffness of the stator along the length of the progressive cavity pump.
 9. The assembly according to claim 1, wherein: the stator comprises a lower section, an intermediate section, and an upper section; each of the lower, intermediate, and upper sections has a reservoir containing a pressure fluid that is isolated from fluid communication with well fluid in the axial cavity; the pressure fluid in the reservoir in the lower section is isolated from fluid communication with the pressure fluid in the reservoir in the intermediate section and the pressure fluid in the reservoir in the upper section, and the pressure fluid in the reservoir in the intermediate section is isolated from fluid communication with the pressure fluid in the reservoir in the upper section; a reservoir pump located exterior of the progressive cavity pump; lower, intermediate and upper fluid lines, leading from the reservoir in the lower, intermediate and upper sections, respectively, to the reservoir pump, each of the fluid lines being blocked from fluid communication with each other, lower, intermediate and upper valves in the lower, intermediate and upper fluid lines, respectively; and a controller that senses a flow rate from the progressive cavity pump and a torque of the motor and controls the reservoir pump and the valves in response so as to be able to apply a different fluid pressure to the reservoir in the lower section from the reservoir in the intermediate section and from the reservoir in the upper section.
 10. A well pump assembly, comprising: a progressive cavity pump having a stator with an elastomeric inner portion, the stator having an axis, an axial cavity with an internal helical profile, and a rotor with an external helical profile positioned within the axial cavity; a motor operatively coupled to the progressive cavity pump for rotating the rotor when supplied with power; at least one elongated reservoir in and extending along a length of the stator containing a pressure fluid; a reservoir pump located exterior of the progressive cavity pump for selectively increasing and decreasing a pressure of the pressure fluid in the reservoir; and wherein increasing the pressure increases a cross sectional dimension of the axial cavity of the stator.
 11. The assembly according to claim 10, further comprising: a valve located in a fluid line between the reservoir pump and the reservoir; and a controller that senses operating conditions of the well pump assembly and controls the valve and the reservoir pump in response.
 12. The assembly according to claim 10, further comprising: a valve located in a fluid line between the reservoir pump and the reservoir; and a controller that senses a torque of the motor and a flow rate of the progressive cavity pump and controls the valve and the reservoir pump in response.
 13. The assembly according to claim 10, wherein the reservoir extends substantially an entire length of the stator.
 14. The assembly according to claim 10, wherein: the at least one reservoir comprises a lower and an upper reservoir, the pressure fluid in the lower reservoir being isolated from fluid communication with the pressure fluid in the upper reservoir; a lower pressure fluid line leading from the lower reservoir to the reservoir pump; an upper pressure fluid line leading from the upper reservoir to the reservoir pump; a lower valve in the lower pressure fluid line, and an upper valve in the upper pressure fluid line, enabling the reservoir pump to apply a different pressure to the pressure fluid in the lower reservoir than the pressure of the pressure fluid in the upper reservoir.
 15. The assembly according to claim 10, wherein said at least one reservoir comprises at least two reservoirs spaced apart from each other on opposite sides of the axis.
 16. A method of pumping fluid from a well, comprising: (a) providing a progressive cavity pump assembly having a progressive cavity pump having a stator with an elastomeric inner portion, the stator having an axis, an axial cavity with an internal helical profile, a rotor with an external helical profile positioned within the axial cavity, and a motor operatively coupled to the progressive cavity pump; (b) installing the pump assembly in a well and operating the motor to rotate the rotor, and (c) while the pump assembly is operating, selectively increasing and decreasing a stiffness of the stator.
 17. The method according to claim 16, wherein step (c) comprises selectively increasing and decreasing a cross sectional area of the axial cavity in the stator.
 18. The method according to claim 16, wherein: step (a) comprises providing a reservoir in the stator containing a pressure fluid that is isolated from well fluid in the axial cavity; and step (c) comprises selectively increasing and decreasing a pressure of the pressure fluid in the reservoir.
 19. The method according to claim 16, wherein step (c) comprises varying a stiffness of a lower portion of the stator relative to a stiffness of an upper portion of the stator.
 20. The method according to claim 16, wherein: step (a) comprises providing a reservoir within the stator, placing a magneto-rheological fluid (MR fluid) within the reservoir; and step (c) comprises generating an electromagnetic field within the MR fluid to selectively increase and decrease a viscosity of the MR fluid. 